The present invention relates to an OFDM transmitting and receiving apparatus and its method for use in communication equipment of a digital radio communication system using OFDM.
In recent years, in digital radio communication system, the use of OFDM (Orthogonal Frequency Division Multiplexing), which is strong against noise and multipath interference, has been considered.
The following will explain signal processing of a conventional OFDM transmitting and receiving apparatus with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the conventional OFDM transmitting and receiving apparatus. It is assumed that the number of subcarriers is four in FIG. 1.
First, in a transmitting side, transmitting signals that have been primary modulated (MOD-S) are parallel-converted to the number of subcarriers, that is, four subcarriers A to D by an S/P (Serial-Parallel) converter 11.
Transmitting signals of the respective subcarries A to D are mapped by mapping circuits 12 to 15, the mapped signals are subjected to inverse fast Fourier transform by an IFFT circuit 16, and they are converted to analog signals by a D/A (Digital/Analog) converter 17. Then, the resultant signals (TR-S) are amplified; thereafter they are radio transmitted from a transmission antenna (not shown).
In addition, in a receiving side, signals (RE-S) received by a reception antenna (not shown) are converted to digital signals by an A/D (Analog/Digital) converter 51, and the converted digital signals are subjected to inverse fast Fourier transform by an FFT (Fast Fourier Transform) circuit 52.
The received signals of subcarriers A to D output from FFT circuits 52 are subjected to detection processing by detectors 53 to 56, respectively, and the resultant signals are demodulated by binary determination of determining units 57 to 60, and the modulated signals are converted to signals (DEM-S) of one channel by a P/S (Parallel-Serial) converter 61.
FIGS. 2A and 2B are views showing a spectrum of the conventional OFDM transmitting and receiving apparatus. FIG. 2A shows a signal spectrum at a signal transmitting time of a base station, and FIG. 2B shows a signal spectrum at a signal receiving time of a partner station. As shown in FIGS. 2A and 2B, an error in reception power occurs by subcarriers under a multipath environment even if transmission power remains constant.
In the conventional OFDM transmitting and receiving apparatus, since reception power does not remain constant, errors concentrate on subcarries wherein reception power has dropped, and this causes a problem in which an error rate characteristic is deteriorated.
It is an object of the present invention is to provide an OFDM transmitting and receiving apparatus and OFDM transmitting and receiving method wherein power between subcarriers can be maintained substantially constant at a receiving time so as to improve an error rate characteristic.
The above object can be achieved by performing such a signal transmission wherein a weight is assigned to power every subcarrier so that power between subcarriers becomes constant at a receiving time and transmission power is differentiated in OFDM transmission.